Roll-to-roll compatible flexible polymer solar cells incorporating a water-based solution-processable silver back electrode with low annealing temperature

The solution-based deposition of the metal back electrode in inverted polymer solar cells (PSCs) using roll-to-roll (R2R) compatible processing technologies is considered one of the crucial issues towards the upscaling of PSC technology, as it may allow the full exploitation of the high through-put and prospective low-costs envisaged by the R2R fabrication approach. In this work, a water-based solution-processable silver ink formulation with low annealing temperature was developed to be used as precursor for the fabrication of the metallic back-electrode in flexible inverted PSC devices fabricated by means of R2R-compatible printing techniques. In order to investigate the effect of the deposition of such reactive silver ink on the underlying PSC layers, different back-electrode architectures were investigated and thoroughly characterized. In addition, the influence of the thickness of the hole-transporting poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) buffer layer on the functional performance of the PSC devices was also investigated and an optimized combination of back-electrode architecture and PEDOT:PSS thickness was found, that also allowed to obtain semi-transparent PSC devices. The results of this study demonstrate the possibility to employ R2R-compatible processing techniques for the deposition of the metallic back-electrode in flexible inverted PSCs from a solution-processable water-based reactive silver ink formulation characterized by low-annealing temperature, and provide useful insights into the key role played by the hole-transporting buffer layer in the realization of fully functional flexible PSC devices.